For convenience the invention will be discussed, by way of example only, with reference to photographic solutions used in black and white processing.
Photographic material, in sheet or roll film form, is processed in several stages, including chemical development, fixing of the image, washing and drying. The role of the photographic fixing solution is to form soluble salts of any unexposed silver halide grains in the emulsion of the sensitized material. As more film is processed, the fixing solution becomes seasoned with soluble silver ion complexes. These complexes reduce the ability of the solution to fix the image, and may affect its final quality. Ultimately, in some instances the solution could become too loaded with silver and it would be necessary to replace it with a totally fresh solution. However, environmental legislation is increasingly putting stricter limitations on the disposal of waste material bearing silver. Consequently, attention is increasingly being paid to safe and efficient recovery of the silver, and it is known to do this electrolytically, either by recovery of silver from the effluent, which is then disposed of, or by in-line treatment in which silver-bearing solutions are withdrawn from a processing tank, passed through the electrolytic cell and returned to the tank. The advantages of in-line electrolytic recovery of the silver include:
(i) the lifetime of the fixing solution can be extended, PA1 (ii) the rate of fixing of the image can be increased, PA1 (iii) the rate of replenishment of the solution with fresh chemicals can be reduced, PA1 (iv) treatment of the effluent from the photographic processing is facilitated, PA1 (v) the value of the silver recovered is economically worthwhile, and PA1 (vi) reduced carryover of silver into the wash, with consequent lower silver concentration in the wash effluent. PA1 (i) film exposure, and thus the proportion of silver that is removed by the fixer, PA1 (ii) film type, and thus the quantity of silver (the coated mass) available for development and fixing, PA1 (iii) film throughput, i.e. how much film is processed per hour, PA1 (iv) processor type, and thus (a) the amount of solution that is carried into the fixing stage from the preceding development stage, and (b) the amount of oxidation that takes place, PA1 (v) the chemical composition of replenisher solution used in the various stages of the processing, and PA1 (vi) the rate at which the processing solutions are replenished.
As with any electrochemical process, however, poor control of the recovery of silver can do more harm than good. When a silver recovery cell is operated efficiently, the only cathodic reaction that occurs is the reduction of silver ions to silver metal, and this is controlled by the potential at this electrode. If too high a potential is applied, then side reactions can occur which lead to the production of unwanted by-products, for example silver sulphide can be formed as a fine precipitate in the solution (sulphiding). The recovery of the silver is often, therefore, a compromise between high rates of silver plating at higher currents, and consequentially at higher potentials, and safe operation. Large scale silver recovery units commercially available employ a third electrode (most commonly a reference electrode, but it may be a pH electrode) or a silver sensor, in order to maintain the efficiency of the operation and to avoid unwanted side reactions. However, these components increase the cost, and problems can arise with calibration of the equipment and electrical drift of the settings. It is possible, however, with the reference electrode, for example, to limit the cathode potential such that the potential for the formation of silver sulphide is not exceeded under any operating condition. EP-B-0598144 employs a third, pH, electrode and the potentials of the three electrodes are controlled so as to avoid sulphiding. In addition to the disadvantage of cost of such a three-electrode system, the maximum rate of removal of silver is itself limited by the fact that the potential of the cathode is kept constant.
The generally cheaper two electrode control system relies on a knowledge of the cell currents and voltages to control the process. The most common method is to use a threshold level beyond which (above which for voltage, or below which for current) it is deemed no longer suitable to recover further silver. For example, when silver is recovered at a constant current, the plating voltage rises as the concentration of the silver in the solution falls--the voltage is reflecting both a change of conductivity in the solution and a change of the potentials of the cathode and anode. A disadvantage to this control method is that the threshold level that is chosen for switch off is not necessarily a suitable or even safe level for switching off under all operating conditions. This problem is exacerbated by the fact that each processor to which silver recovery is attached has a specific combination of operating parameters reflecting the variability in the concentration of the constituents of the solution arising from variation in:
The specific combination of the above variables used by the operator of a given processing system is known as the `operator profile`.
The voltage necessary to supply a certain current through a fixer solution at a given silver concentration, for example, will depend strongly on the pH of the solution, the concentration of the sulphite and/or thiosulphate in the solution, the temperature of the solution, and the rate at which it flows through the cell.
U.S. Pat. No. 4,619,749 overcomes the problems associated with setting reference voltage control thresholds which are valid for a wide variety of different solutions, by using calibration solutions with high and low silver concentration. The disadvantage of this approach is that the operator must obtain the reference solutions that are characteristic of his normal operating conditions, and then perform the calibration. GB-A-1500748 overcomes the problems associated with solution variability and the choice of suitable operating conditions common to two electrode systems, by employing a second electrolytic cell as a reference. The disadvantage of such a control system, however, is that it is inconvenient for the operator to use since the test cell has to be set up and employed for every solution from which it is desired to remove the silver. U.S. Pat. No. 3,925,184 employs a work counting method, which takes account of the silver entering the system as a result of film input and the silver leaving the system through plating reactions. The silver ion concentration in the fixer solution is estimated and a suitable current, based on a known relationship, is applied to the electrolytic cell. The disadvantage of this control method is that the amount of silver entering into the system has to be known accurately. In U.S. Pat. No. 3,980,538, a similar work counting method is employed in which the magnitude of the control current in the electrolytic cell is governed by the amount of charge on a capacitor that is intended to correspond to the quantity of silver present in the solution.
U.S. Pat. No. 4,776,931 discloses recovering metals from solutions by applying an intermittent plating voltage until the current drawn by the solution exceeds a predetermined threshold value above which the recovery system operates. U.S. Pat. No. 5,310,466 similarly operates using threshold values. Each of these systems has the disadvantages set out above of variability introduced by the operator.
U.S. Pat. No. 4,018,658 discloses a silver recovery system in which the voltage across the electrodes and the current passing between them are monitored, and the voltage is adjusted using a feedback loop so as to achieve the optimum current density. The system employs a predetermined voltage-current characteristic and is thus not able to adapt to any variation in the solution of the electrolytic cell.
EP-A-0201837 discloses a silver recovery process in which the electrolytic cell is operated at the plateau of the potential difference/current curve, that is to say at that point where the current is determined by the speed of diffusion of silver to the cathode surface. EP-A-0754780 is said to be an improvement on this system, in which that condition, referred to as the diffusion limitation current, is ascertained and the cell is then operated at a current density which is lower than the diffusion limitation current density. Amongst the ways proposed to determine the diffusion limitation current density, is mentioned the periodic measurement of a current-potential characteristic of the cell at a given silver concentration under de-silvering conditions. One such characteristic, although not a preferred one, is specified as being the curve of current versus the potential difference between the anode and the cathode, with a diffusion limitation current being determined by identifying the cell current when the second derivative of the current-potential characteristic is zero and the first derivative is minimal. The disadvantage of this system is the difficulty of obtaining a sufficiently accurate measurement of the diffusion-limited current by such a method.